The present invention relates to improvements in a slip control system which converges a slip rotation speed between input and output elements of a torque converter at a target value, and more particularly to a slip control system which suitably functions even during a transient period from the non slip-control range to a slip control range.
Generally, a torque converter has merits such as a torque fluctuation absorbing function and a torque multiplying function due to a fluid power transmission, although a transmission efficiency of the fluid power transmission is lower than that of a power transmission by a direct mechanical connection. In order to improve the transmission efficiency of the torque converter, most torque converters employ lockup clutches for directly connecting input and output elements of the torque converter when the vehicle is put in a running condition where the torque fluctuation absorbing function or the torque multiplying function are not required. Further, some of lockup torque converters have been proposed to execute a slip control of a lockup clutch. Such a slip control for a torque converter is generally arranged to determine a target slip rotation speed according to a vehicle running condition and to control an engagement force of the lockup clutch so as to adjust an actual slip rotation speed to the target slip rotation speed.